The present specification generally relates to feedback control of downlink connections in carrier aggregation scenarios, especially in scenarios where time division duplex (TDD) is deployed.
In Long Term Evolution (LTE) Rel-10, the physical uplink control channel (PUCCH) format 3 only supports up to 20 ACK/NAK bits per feedback transmission. Hence, PUCCH format 3 can only support ACK/NAK feedback for TDD configuration 5 with up to two component carriers (CC), since in case of more CCs, more than 20 ACK/NAK bits would arise. In case of two CCs, a maximum number of ACK/NAK bits of 36 may arise. Hence, in case of two CCs (if the total number of ACK/NAK bits exceeds 20), spatial bundling is applied. At that, spatial (domain) bundling means that if two (more than one) codewords are transmitted per subframe, the ACK/NAK bits corresponding to each of the codewords are combined via an AND operation and the resulting bit is the spatial bundled ACK/NACK bit. In LTE Rel-10 in relation to PUCCH format 3 for TDD configuration 5, no time or carrier domain bundling is supported.
In LTE Rel-11, a user equipment (UE) configured with TDD configuration 5 and more than 2 CCs will be required to provide downlink (DL) peak data rates. In recent RAN1 meetings, the necessity of support of the ACK/NAK feedback in this case is raised and different bundling schemes to support the ACK/NAK feedback are discussed.
The following table illustrates the maximum number of ACK/NAK bits that may arise in the case of TDD configuration 5. It is to be noted that according to TDD configuration 5, a ratio of uplink (UL) subframes to DL subframes is set to 9:1. In particular, subframe 0 is set to DL, subframe 1 is set to DL/special subframe (guard periods), subframe 2 is set to UL, and subframes 3 to 9 are set to DL. As is derivable from the following table, in case of TDD configuration 5 and 5 CCs, the maximum number of ACK/NAK bits that may arise is 45 after spatial domain bundling.
The number of CCs2 CCs3 CCs4 CCs5 CCsMaximum number of A/N36547290bits without bundlingMaximum number of A/N18273645bits with spatial domainbundlingHence, the problem arises that in a deployment with more than 2 CCs which uses TDD configuration 5, the required ACK/NAK bits, even if spatial bundling is applied, exceed 20, which is the maximum number of ACK/NAK bits that PUCCH format 3 can carry.
Hence, there is a need to provide for ACK/NAK bit bundling in carrier aggregation scenarios.